The B7 family consists of structurally related protein ligands, which bind to the CD28 family of receptors on lymphocytes to regulate the immune response via co-stimulatory or co-inhibitory signals. It is the engagement of the T-cell receptor with specific cell surface antigen that initiates the first step of lymphocyte activation, but an additional signal delivered simultaneously by B7 ligands determines the ultimate fate of the immune response. B7x (B7-H4 or B7S1), a poorly characterized B7 family member, is expressed on immune cells, non- lymphoid tissues, and nearly all human tumor types and acts to down-regulate immune responses by inhibiting cell cycle progression, proliferation, and pro-inflammatory cytokine production of activated CD4+ and CD8+ T cells. B7x has been observed in cancer patients of nearly all types and this expression correlates positively with cancer staging, probability of recurrence, and negatively with survival, leading ultimately to poor clinicl outcomes. Currently, the mechanism B7x utilizes to generate inhibition of anti-tumor immunity remains elusive. The goal of this project is to reveal the physiological role of tumor expressed B7x in murine cancer models and generate anti-tumor immunity with a B7x-specific monoclonal antibody. We propose to accomplish these objectives by injecting B7x-positive and B7x-negative murine tumor cell lines into mice and monitoring the tumor growth and evolution of the immune reveal the physiological role of tumor expressed B7x in murine cancer models and generate anti-tumor immunity with a B7x-specific monoclonal antibody. We propose to accomplish these objectives by injecting B7x-positive and B7x-negative murine tumor cell lines into mice and monitoring the tumor growth and evolution of the immune response between both groups of mice. response between both groups of mice. We will then create a B7x-specific transgenic pancreatic cancer mouse model to evaluate the role of B7x in spontaneous tumorigenesis. Furthermore, we plan to evaluate the novelty of B7x as a target for immunotherapy by evaluating the efficacy of our newly generated anti-B7x specific monoclonal antibodies in generating anti- tumor immunity. We believe this project fully aligns with the missions of the National Cancer Institute because it will provide much needed insight into the mechanisms of B7x immune evasion in the cancer-associated pathway and highlights B7x as a viable and novel therapeutic target to increase overall improvement in patient outcome.